1. Field of the Invention
The present invention generally relates to an ATM (asynchronous transfer mode) switching device capable of switching one of lines to another of the lines, and in particular to a header conversion technique for the line switching.
2. Description of the Related Art
In general, an ATM switching device having line protection capability is provided with a header converter and a header conversion table, which are used to switch a working line to a reserved line. More specifically, a plurality of line interfaces are connected to a multiplexer, which multiplexes incoming fixed-length packets (cells) received from the respective line interfaces to produce a sequence of cells each having the incoming line number thereof attached therewith. When receiving the sequence of cells from the multiplexer, the header converter reads the incoming line number and VPI/VCI (Virtual Path Identifier/Virtual Channel Identifier) for each cell and uses them as a key to search the header conversion table for output information necessary for a switch fabric to forward the cell to an appropriate output port thereof. The header converter converts the header of the cell using the found output information. Such output information includes an outgoing line number, outgoing routing information VPI/VCI, and control information for controlling the quality of service for each cell flow. The header conversion technique as described above has been disclosed in Japanese Patent Application Unexamined Publication Nos. 7-74747 and 10-79747.
In the case of redundant system architecture, however, two memory areas used for respective ones of working system and reserved system are needed to store the same information in the header conversion table, resulting in the increased amount of hardware and the increased amount of memory for header conversion table.
As shown in FIG. 1, for example, a 1+1 redundant system having #0 (working) and #1 (reserved) incoming lines includes a header conversion table storing necessary information for respective ones of #0 and #1 incoming lines. When the working line normally functions, the header converter accesses a set of information for the #0 incoming line to obtain necessary information for the switch fabric to forward the cell to an appropriate output port thereof. If the working line is switched to the reserved line due to occurrence of a failure on the #0 system, then a set of information to be accessed is changed from the #0 incoming line to the #1 incoming line.
Therefore, if the set of information for the #0 incoming line is not identical to that for the #1 incoming line, then the line switching cannot be successfully performed. It is necessary to always store the same set of information for the #0 and #1 incoming lines in the header conversion table.
It is the same with the case of N:1 redundant system having N working incoming lines and a single reserved incoming line. In this case, it is further necessary to copy the latest information after a failure has occurred on the working incoming line to a memory area for the reserved incoming line. Since the table duplication is needed after the occurrence of a failure, it is not possible to perform the line switching immediately after the failure occurs and therefore the increased speed of line switching cannot be achieved.